Dog "bully" - dog smells at another dog's rear

It's partly aggressive behaviour because the little dog obviously didn't want it's behind to be smelled by the other dogs. Unless it just ran away playfully as some dogs would? It depends on the situation and I can't tell without having been there.

Anyhow like the other posters commented it is perfectly natural for dogs to sniff each others behinds as it's a way of identifying one another and in some cases greeting each other. Normally it's totally harmless.